1. Field of the Invention
This invention relates generally to switch-mode power converters and more particularly it provides a simple drive circuit with an enable function providing isolation and having high performance in topologies using synchronous rectification.
2. Background Discussion
A switch-mode power converter is a circuit that uses an inductor, a transformer, or a capacitor, or some combination, as energy storage elements to transfer energy from an input source to an output load in discrete pulses. Additional circuitry is added to maintain a constant voltage within the load limits of the circuit. The basic circuit can be configured to step up (boost), step down (buck), or invert output voltage with respect to input voltage. Using a transformer allows the output voltage to be electrically isolated from the input voltage.
Switch-mode converters have changed very little over the past 15 years, most using Schottky diodes to rectify their output. However, newer challenges in the industry for dc/dc power supply designers demand lower voltages required by digital circuits, and also higher frequencies. Since converters using Schottky diodes for rectification experience a large forward voltage drop relative to the output voltage, their efficiency is generally relatively low. Lower efficiencies result in more dissipated heat that has to be removed using a heat sink, which takes up space. A dramatic increase in converter efficiency has been realized by replacing the Schottky diodes with “synchronous rectifiers” configured in practice with MOSFET transistors. Synchronous rectifiers are not new, but they have previously been too expensive to justify, primarily due to high ON resistance. However, as costs fall and performance improves, synchronous rectifiers have quickly become a viable component, especially for low voltage converters.
Using self-driven synchronous rectifiers in various converter topologies is very attractive and popular because there is no need for additional isolation between drive signals. It has the advantage of simplicity. However, it has the disadvantage of cross conduction between synchronous rectifiers and primary side switches, as well as reverse recovery current of the parasitic anti-parallel diode of the MOSFET used for synchronous rectification. In order to minimize these shoot-through currents, an inductance (or saturable inductor) is usually placed in series with the synchronous rectifier. While this may be a solution for lower switching frequencies, for example, 100 kHz-200 kHz, it is not suitable for higher switching frequencies (200 kHz and above). Especially at switching frequencies of 300-400 kHz this is not an optimum solution. The reason for this is that increased inductance in series with a synchronous rectifier reduces the effective duty cycle on the secondary side of the power transformer due to slower di/dt of the secondary current. As a result, more voltage head-room is required in the power transformer implying a smaller effective turns ratio and consequently a lower efficiency. A second reason why self-driven synchronous rectification is not suitable for higher switching frequencies is the potential loss due to reverse recovery current in the body diode of the synchronous rectifiers (MOSFETs) and increased turn-on current in the primary side switches (usually MOSFETs).
A previous improvement has been to use direct drive for synchronous rectifiers with well controlled timing between drive signals for the main switches (primary side) and synchronous rectifiers (secondary side). This solution thus allows for very efficient operation of the synchronous rectifiers even at high switching frequencies. Yet another benefit of direct driven synchronous rectifiers is that the drive voltage (gate to source) is constant and independent of input voltage, which further improves efficiency over a wide input voltage range.
An example of the above prior art is set forth in U.S. Pat. No. 5,907,481. However, the invention in this patent provides only signals for drivers for the primary side switches and uses additional logic for delaying drive waveforms (R-C-D networks and logic gates). The '481 apparatus uses an isolation drive transformer for a logic signal only to control operation of the primary switches. It does not use a drive transformer to power the primary switches but rather uses a separate circuit to provide power and delays.
It is necessary to provide delays between drive signals for primary side switches and secondary side switches (synchronous rectifiers) in order to avoid cross conduction (simultaneous conduction which would result in a short circuit). When power converters are operated at lower switching frequencies (for example, 100 kHz), cross conduction of the switches can be acceptable since the percentage of the time during which cross conduction occurs relative to the switching period is small (typically about 40 ns/10 μs). Also, a transformer designed to operate at lower frequencies will have a larger leakage inductance, which will reduce cross conduction currents. In the case of higher switching frequencies (above 100 kHz), the cross conduction ratio becomes more unacceptable (about 40 ns/2 μs for a 500 kHz switching frequency). Also for higher switching frequencies, the leakage inductance in the transformer as well as in the entire power stage should be minimized for higher efficiency. Consequently, currents due to cross conduction time can become significant and degrade overall converter efficiency and increase heating of the power components significantly.